The broad goal of this application is to investigate the fundamental requirements for effective use of tools. The working hypothesis is that the central nervous system may take advantage of the apparent mechanical behavior of the neuro-muscular system (e.g., muscle elastic and viscous properties, skeletal geometry and inertia) to circumvent some of the major computational problems of controlling posture, movement and interactive tool-using behavior. The proposed research will study, kinematically-constrained motions, (e.g. opening a door) in which the hand may move only in certain directions. Experiments are proposed to evaluate the ability to perform a simple but highly-informative task- turning a crank (a handle mounted on a rod constrained to rotate about a pivot point). The experiments are to be augmented by and coordinated with theoretical studies. A detailed computer simulation of the upper extremity performing constrained motions will be developed. The work will also test a novel, biologically-motivated approach to controlling externally-powered arm prostheses which has been devised to enhance an amputee's ability to use tools. The controller makes a prosthesis mimic aspects of natural arm behavior: the response to external forces varies with co-activation of agonist and antagonist muscles while differential activation generates motion. The research method features an unique computer-controlled arm prosthesis. It can be programmed to mimic the behavior of any self- contained prothesis-existing or proposed. With the facility experiments can be performed which are not otherwise feasible with human subjects. Key aspects of the behavior of the (artificial) forearm can be manipulate directly-the way it responds to the human, the way it responds to the external world. Parallel studies of able-bodied subjects are planned to complement the experiments with amputees using the programmable prosthesis. Including amputees using powered arm prostheses in the study permits unique non-invasive perturbation experiments on human subjects. At the same time, a comparative study of able-bodied and amputee constrained- motion behavior will provide quantitative design specification for prosthesis control systems.